The Messenger of Allaah said: "The best supplication is the supplication on the day of Arafat."
At Tirmidhi #3585, graded as Hasan by Shaykh al Albaanee.
The Prophet said – "The best du’aa, is the du’aa of ‘Arafah. The best of that which myself and the Prophets before me have said is; "Laa Ilaaha Illallaahu Wahdahu laa shareekalahu, lahul Mulk wa lahul Hamd wa Huwa ‘Alaa Kulli Shay-in Qadeer." None has the right to be worshiped except Allaah, who is Alone without partners. For Him is the Dominion and Praise. He is Capable of doing all things."
Al Musnad of Imaam Ahmad, Sunan at Tirmidhee, declared good by Shaykh al Albaanee.